1. Field of the Invention
The invention relates to an apparatus for reproducing information from an optically readable record carrier on which information is stored in the form of a track of optically detectable pit areas which alternate with intermediate land areas, which apparatus comprises:
an optical system for projecting a light beam on the record carrier,
an optical detector for detecting the light beam which has been modulated by the record carrier and generating a detection signal which is determined by said modulation, and
track-loss detection means for generating a track-loss signal which is an indication of a condition in which the spot produced by the light beam is positioned off the track to a specific extent.
2. Description of the Related Art
Apparatus of this general kind is utilized in the "compact disc digital audio player" which is commercially available from N. V. Philips' Gloeilampenfabrieken under the type number CD 100. In this apparatus the amplitude of the detection signal, after this detection signal has been filtered in a high-pass filter in order to remove low-frequency variations, is used for detecting a loss of track. However, detecting only the amplitude of the high-frequency signal has the disadvantage that decreases in signal amplitude due to of other causes, such as fingermarks on the record carrier, may be interpreted as a loss of track. In order to preclude this, variations of the average value of the detection signal are also detected. Nevertheless, the use of a high-pass filter has the disadvantage that pulse-shaped signal variations, such as produced by a signal drop-out due to a defect in the disc, result in an error propagation due to the impulse response of said filter.
Applicant's copending U.S. application Ser. No. 589,386, filed concurrently herewith, relates to apparatus similar to that described herein but does not address the track-loss problem to which the present invention is directed.